1. Field of the Invention
The present invention relates to an apparatus and a method for controlling focus of laser beam in an optical disk and, more particularly, to an optical disk drive, a focus control apparatus, and a focus control method having the function of controlling a focus jump operation performed on a multilayer optical disk.
2. Description of Background Art
Conventionally, as optical disks driven by an optical disk drive (reproducing or writing apparatus), a CD (Compact Disk) and a DVD (Digital Versatile Disk) are used. The DVD has a multilayer structure having two or more recording layers for recording a large amount of information.
FIG. 6 shows an example of the structure of a DVD having two recording layers. A DVD 100 of FIG. 6 has a structure that a disk in which a recording layer 102 is formed on a substrate 101 and a disk in which a recording layer 104 is formed on a substrate 103 are adhered to each other via a space layer 105. Substrates 101 and 103 are made of a transparent material such as a polycarbonate resin. Space layer 105 is made of a resin material for bonding the substrates, and the resin material has a light transmitting property.
In a disk drive for DVD 100, a laser beam is emitted from an optical pickup 1 which will be described later onto tracks on recording layer 102 or 104 of DVD 100 rotated by a not-shown spindle motor and reflection light is detected, thereby reading (reproducing) data.
To perform reproducing operation by using a laser beam, the spot of a laser beam has to be maintained in a focus state on recording layer 102 or 104. Consequently, a focus servo mechanism for controlling a focus state by moving a not-shown objective lens as an output end of the laser beam so as to be close/apart to/from DVD 100 is mounted.
The focus servo mechanism usually includes a two-function driver constructed by a focus driver for moving the objective lens so as to close/apart to/from DVD 100 and a tracking driver, and a focus servo circuit system. The focus servo circuit system generates a focus error signal FE from reflection light information from DVD 100, operates a focus drive signal FD on the basis of focus error signal FE, and applies focus drive signal FD to a not-shown focus coil of the two-function driver. That is, the focus servo mechanism is constructed as a feedback control system.
Referring to FIG. 7, a timing of turning on the focus servo system on the basis of focus error signal FE will be described. Focus error signal FE is a signal indicative of a deviation amount from a focus state on a recording face of the beam spot of a laser beam. FIG. 7 schematically shows a focus state by optical pickup 1 on recording layers 102 and 104 of FIG. 6 and the waveform of focus error signal FE with lapse of time T.
As already well known, the range in which a laser beam can be led to a focus state on the basis of focus error signal FE is a very narrow range in which an S curve is observed as focus error signal FE. Consequently, to execute focus serve excellently, timings to turn on a focus servo loop are important. The timing of turning on the focus servo is as follows. When focus error signal FE is observed and the position of the objective lens is in a range, the S curve of focus error signal FE is observed. At timings T1 and T2 in FIG. 7 corresponding to timings when the S curve becomes linear (or zero cross timings), the focus servo loop is turned on. FIG. 7 shows that at timing T1, a focus state is obtained in recording layer 104, after that, an operation of jumping a laser beam to recording layer 102 is performed and, at the next timing T2, a focus state is achieved in recording layer 102.
To move the focal point of a laser beam from a state were a laser beam from optical pickup 1 is focused in recording layer 102 (or recording layer 104) to recording layer 104 (or recording layer 102) during reproduction will be called a focus jump. In the case of making such a focus jump, a pulse signal constructed by an acceleration signal or a deceleration signal is applied to the focus servo loop to make a laser beam from optical pickup 1 make a jump to a new recording layer. After that, a focus servo is applied so as to lower the level of focus error signal FE on the face of the new recording layer.
The flowchart of FIG. 8 shows a control procedure of a conventional focus jumping operation. FIGS. 9 and 10 show examples of waveforms of focus error signal FE and focus drive signal FD detected during the focus jumping operation of FIG. 8 together with lapse of time T. A conventional focus jumping operation will be described with reference to FIGS. 8 to 10.
First, during a scan of tracks in a recording layer in DVD 100 with optical pickup 1, when an instruction of a focus jump to another recording layer is detected on the basis of a signal read by the scan, on the basis of the detected jump instruction, a focus jumping operation starts. It is assumed here that a jump to recording layer 102 is instructed during scan of tracks in recording layer 104 in FIG. 7.
In a period in which a laser beam from optical pickup 1 is in a focus state in recording layer 104 (first layer) (period T10 in FIG. 9), focus error signal FE maintains a focus level. When the focus jump instruction is detected at timing P1 in FIG. 9, the focus jump is started (step (hereinbelow, referred to as S) 31).
First, an acceleration pulse PL1 is applied to focus drive signal FD (S32), so that optical pickup 1 starts moving to make the laser beam jump to recording layer 102 (second layer) by a focus servo. At this time, in parallel with the movement, focus error signal FE is detected and whether or not the level of detected focus error signal FE has reached a predetermined acceleration complete level ACL shown in FIG. 9, that is, whether or not the level of focus error signal FE has reached the level indicative of completion of movement to recording layer 102 of the laser beam from optical pickup 1 (S33) is determined (S33).
When it is determined that the level of focus error signal FE has reached acceleration complete level ACL, at timing P2 in FIG. 9, focus drive signal FD is held at 0 to finish acceleration (S34). In period T20 in FIG. 9 after completion of acceleration, the laser beam from optical pickup 1 continues moving due to inertia. In period T20, the laser beam is in a focus in a semitransparent layer, so that light is not reflected and focus error signal FE is not detected.
After that, whether focus error signal FE has reached a predetermined deceleration commence level DCL or not is determined (S35). If it is determined that focus error signal FE has not reached the level (NO in S35), whether predetermined deceleration commence time-out period TT has elapsed after completion of acceleration or not is determined (S36). If the deceleration commence time-out period TT has not elapsed yet (NO in S36), the program returns to S35 and the process is repeated.
On the other hand, when it is determined that deceleration commence time-out period TT has elapsed (YES in S36), a deceleration pulse PL2 is outputted for predetermined time to focus drive signal FD and the focus servo system is stopped (S37 and S38). By the operation, the focus jump of the laser beam of optical pickup 1 to recording layer 102 is completed, focus error signal FE reaches a focus level, and the laser beam of optical pickup 1 is in a focus in recording layer 102.
According to FIGS. 8 and 9, the focus servo is controlled so as to lead a laser beam to a focus state on the basis of focus error signal FE. However, since focus error signal FE includes an error caused by variations in intervals in a plurality of recording layers and variations in transmittance of light according to materials of layers constructing DVD 100, it is difficult to stably always make an accurate focus jump so as to bring light into a focus on a target recording face.
Consequently, in the conventional focus jump control operation, as shown in FIG. 10, in the case where the level of focus error signal FE of the recording layer as a destination of the focus jump is too low and cannot reach deceleration commence level DCL, deceleration pulse PL2 cannot be applied in deceleration commence time-out period TT. Due to this, deceleration pulse PL2 is applied when the laser beam is positioned in a region which is not the recording layer as a jump destination, so that the focus jump cannot be accurately made.
Techniques regarding focus jump control other than the above-described focus jump operation are disclosed in Japanese Patent laying-Open Nos. 11-203685, 11-353657, 2000-298846, and 2000-353324.
Japanese Patent laying-Open No. 11-203685 teaches a procedure of making a safe jump by applying a brake before a focal point in a focus jump operation. In the publication, in a disk in which many axial runouts occur, to avoid erroneous operation of a focus jump caused by variations in relative speeds of the disk and a beam spot (lens), a brake is applied by using a differential signal of the focus error signal and a servo is turned on at the next zero-cross point of the focus error signal.
Japanese Patent laying-Open No. 11-353657 discloses a procedure of switching a braking operation according to lapse time from start of a control to a zero-cross point of focus error signal FE.
In Japanese Patent laying-Open No. 2000-353324, movement time of an optical beam from a present position to a position in a destination of a focus jump is measured and, on the basis of the measured movement time, the waveform of a deceleration pulse applied to focus drive signal FD is changed.
However, the conventional techniques do not show a procedure for applying an acceleration pulse and a deceleration pulse for a focus jump while monitoring the level of focus error signal FE. Therefore, an error of focus error signal FE caused by variations in intervals in a plurality of recording layers and variations in transmittance of light according to materials of layers constructing a disk cannot be absorbed and a focus jump cannot be made accurately.
Japanese Patent laying-Open No. 2000-298846 teaches a procedure for stably making a focus jump irrespective of variations in the structure of a focus actuator and variations in intervals of a plurality of-recording layers by making the threshold level of focus error signal FE different values in five points. Concretely, by making a threshold at the start of braking operation and a threshold of stop of braking operation different values, detection of the threshold of start of braking operation is facilitated. However, the thresholds are fixed, so that they cannot follow fluctuations in threshold level of focus error signal FE caused by variations in the material of a disk. Consequently, an error of focus error signal FE caused by variations in transmittance of light according to materials of layers constructing a disk cannot be absorbed and a focus jump cannot be made accurately.